Weather stations for home, agricultural, and industrial use have become increasingly numerous. In such stations, two of the most common measurements taken relate to humidity and temperature. Unfortunately, placement of the measuring sensors is critical. Users of such measuring sensors frequently must place the measuring sensors where thermal radiation conditions can and do create measurement error, especially as related to temperature and humidity. In certain shields for weather sensors, low wind speeds (for example fewer than five miles per hour), can cause obvious errors in temperature measurement. These errors can be quite large--in the range of 5.degree. Centigrade.
Because these weather sensors must be placed in locations where incident radiation effects can introduce error into the desired measurement; the prior art has tried to provide the measuring sensor within a protecting shield. These devices are either passive shields, relying on natural ventilation provided by the wind, or active shields having a fan-induced air flow.
Regarding such passive shields, it is known to place weather sensors in cylindrically shaped louvered housings. The louvers are intended to block radiation from affecting the reading of the sensors. At the same time, these louvers allow the naturally occurring wind to ventilate the chamber containing the sensors.
Unfortunately, at very low wind speeds, these passive sensor shields loose their effectiveness. The louvers are heated by solar radiation. When the louvers become hot, their heat is passed on to the contained sensors.
As radiation effects are reversible, both day and night measurements are affected. The night sky is a cold radiation "sink."
It is known to provide aspirated housings for weather sensors. In such shields, a fan typically draws air through the shield past a measuring sensor. The outer wall of the ventilated chamber is not shielded from solar radiation. High aspiration rates are required for the heat to be drawn off.
Alternately, in forced ventilation sensors, it is known to draw air over radiation deflecting louvers to a centrally contained sensor. Unfortunately, when the radiation deflecting louvers become warm, the air being drawn over the louvers is heated. Sensor error can result.